I want my site to serve 1X and 2X images depending on the browser using something like Picturefill, and I don’t want the 1X version to just be automatically generated using a transform - I want to be able to upload separate 1X and 2X versions, and have them be linked together. How can I go about that?

With that set up, after you select 1X images in your main Assets field, you will be able to double-click on them and see your “Hi-Res Image” Assets field in their metadata modals. You can drag your 2X image into that, and it will automatically get saved into a _2x/ subfolder wherever its parent image lives.

From your templates, you can then output <img> tags that point to both images: